During engine non-fueling conditions in which at least one intake valve and one exhaust valve are operating, such as deceleration fuel shut off (DFSO), ambient air may flow through engine cylinders and into the exhaust system. In some examples, an exhaust gas sensor may be utilized to determine ambient humidity during the engine non-fueling conditions. However, due to intake throttle closure during the engine non-fueling condition, a large intake manifold vacuum is generated which can draw in positive crankcase ventilation (PCV) hydrocarbons. As such, even if a PCV port is closed during the DFSO, the vacuum may be sufficiently strong to draw in PCV hydrocarbons through the piston rings. The PCV flow drawn in may be aggravated in an aging engine due to leakage of PCV gases past the piston rings and valves. The ingested hydrocarbons affect the output of the exhaust gas sensor and can confound the humidity measurements. In particular, the hydrocarbon effect leads to a sensor output that overestimates the ambient humidity.
The inventors herein have recognized the above issue and have devised an approach to at least partially address it. Thus, a method for an engine system which includes an exhaust gas sensor is disclosed. In one example, the method includes, during engine non-fueling conditions, where at least one intake valve and one exhaust valve are operating, modulating a reference voltage of an exhaust gas sensor with an intake throttle closed and open; and indicating engine degradation based on PCV flow, the positive crankcase ventilation (PCV) flow based on outputs of the sensor during the modulating. The method may further comprise generating an indication of ambient humidity based on an output of the exhaust gas sensor with the intake throttle closed and the estimated PCV flow. In this way, a more accurate ambient humidity estimate may be achieved and PCV flow may be better estimated and accounted for.
For example, during selected deceleration fuel shut off (DFSO) events, an engine controller may modulate the reference voltage of an intake oxygen sensor to estimate each of an ambient humidity and a PCV flow. The controller may first modulate the reference voltage with the intake throttle open and then re-modulate the voltage with the intake throttle closed. With the intake throttle open, manifold pressure is increased and PCV flow to the intake is reduced. During such conditions, a change in pumping current read at the sensor during the modulating is indicative of an ambient humidity. With the intake throttle closed, manifold pressure is decreased and PCV flow to the intake is increased. During such conditions, a change in pumping current read at the sensor during the modulating is indicative of an ambient humidity as well as an effect of PCV hydrocarbons. By comparing the change in pumping currents estimated with the intake throttle open and closed, an amount of PCV flow received in the engine during closed throttle conditions can be identified. If the PCV port was also closed during the modulating, the PCV flow can be compared to a threshold to identify PCV flow leakage past piston rings and an indication of engine aging and component degradation can be signaled. The ambient humidity can also be further modified based on the learned PCV flow. The more reliable ambient humidity estimate, free of hydrocarbon effects from PCV, can then be used to adjust engine operating parameters without incurring engine control issues.
In this way, the PCV impact on humidity measurement by an exhaust gas oxygen sensor is reduced. By selectively opening the intake throttle during a DFSO when humidity measurement is required, intake manifold vacuum is reduced, lowering the amount of PCV hydrocarbons drawn into the engine. In addition, even if any PCV hydrocarbons are ingested, the increased airflow reduces the PCV concentration sensed by the exhaust gas sensor during the DFSO. As such, the ambient humidity may be determined more accurately and reliably.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.